Electronic clocks with a liquid crystal display (LCD) commonly made up of segments include a quartz crystal oscillator for providing timing pulses and are powered both by solar power and rechargeable battery. These are known as shown by Goetzberger U.S. Pat. No. 4,763,310. Since the battery power is limited, it is desirable to minimize power consumption of the battery by turning off the LCD display when it is not visually observable.
The above patent discloses how to do this by providing a diode connected between a solar cell array and the battery which permits the battery to be recharged by the solar cell array, but prevents the display from drawing energy from the battery. Thus, in effect, the liquid crystal display is only in operation when there is enough light for reading the display. However, the battery still provides power to the internal clock circuit. Thus, energy is saved which is used to power the clock electronics for a longer period of time and results in a substantial increase in battery power reserve.
One difficulty with the foregoing is that since the solar cell array is a current source, the voltage at its terminals, up to its rated terminal voltage, will invariably depend on the electrical load it is driving. In an electronic clock of the present type where LCD segments are being switched to provide different numbers, this causes the load to change and thus a varying voltage will appear between the liquid crystal segments causing display malfunctions or undesirable display effects such as ghost shadows or a dim display. Also, normally, the liquid crystal display requires an alternating voltage to drive its segments and thus requires elaborate circuitry to provide the proper type of voltage. Finally, there is a very delicate balance between capacity of the solar cell to trickle charge the rechargeable battery and the amount of energy needed to be provided by that battery to insure continued and reliable operation of the timing circuitry during nighttime operation.